Dual fuel operation of internal combustion engines, though long known and practiced, is nowadays coming more and more to the fore in view of the growing shortage and cost of liquid fuel, particularly gasoline. Especially in the cases of larger vehicles, such as trucks, motor homes and the like, their innate higher fuel consumption has made the ability to operate them either on liquid or gaseous fuel increasing attractive owing to the relatively plentiful supplies and lower cost of gaseous fuels, such as propane, compared to gasoline.
Currently, several schemes to this end are in practice. Some replace the typical air filter assembly with one which incorporates a fixed venturi arrangement for metering the gaseous fuel. A separate arrangement, exterior of the air cleaner assembly, is usually also fitted for slow-run operation. Some other approaches attempt to incorporate both the main and slow run systems into the air cleaner assembly. These employ, for instance, a diaphragm operated valve which controls a small variable venturi for slow run and a separate fixed large venturi for high speed operation, or a diaphragm operated air valve in conjunction with a fuel valve which together meter both air and fuel throughout the engine speed and load range including slow-run. Examples of these two latter approaches are found in U.S. Pat. Nos. 2,927,848 and 2,983,592, from which it will be readily seen that both are fairly intricate and thus expensive and elaborate to manufacture.
In any event, since all the prior art devices whatever their nature, fit upstream of those means in the engine air intake passage which meter liquid fuel and air in carbureted and fuel injected engines, their paraphernalia of venturis, air valves and the like would improperly throttle air supply and upset functioning of the liquid fuel metering means downstream of air filter assembly when operating on liquid fuel. Hence, the foregoing systems necessarily incorporate some manner by which the incoming air can bypass the gaseous fuel metering and mixing means when the engine operates on liquid fuel. Typically the bypass constitutes a separate or alternate air intake route which is closed off during gaseous fuel operation and opened during liquid fuel operation either manually, as by a handworked cable, or electrically as by a solenoid operated valve. Such a manual approach, of course, requires extra parts and the running of the cable from the engine to the driver's station. The electrical approach, while it can be arranged to operate automatically when the engine is shifted from one fuel to the other, also requires additional, not inexpensive parts and is not always reliable in operation to boot.
Accordingly, the primary object of the present invention is to provide gaseous or liquid fuel operation of an internal combustion engine by means which are simple in structure, inexpensive to produce, efficient in operation, and capable of automatically accommodating either fuel without the need of a separate air bypass route in conjunction with additional manual, electrical or other means for opening and closing the same.